1. Technical Field
Exemplary embodiments of the present disclosure are directed to methods and apparatuses for soft-decision detection in 2×2 MIMO systems, and more particularly to methods and apparatuses for detecting the soft-decision of a signal modulated through an MDCM (Modified Dual-Carrier modulation), one of precoding modulation schemes in a UWB (ultra-wideband) system based on a 2×2 MIMO (multi-input multi-output) MB-OFDM (multi-band orthogonal frequency division multiplexing) scheme.
2. Discussion of the Related Art
An MB-OFDMA UWB communication system employs an MDCM technology, which is a precoding modulation scheme that can improve PER performance and the receive distance. MDCM technology is a modification of a DCM technology that provides higher speed transmission. MDCM technology combines and converts two independent 16-QAM (quadrature amplitude modulation) symbols into two 256-QAM symbols that are carried on two subcarriers which are spaced apart from each other by the maximum distance of an OFDM symbol. MDCM technology allows diversity gains for different subcarriers to be acquired in one frequency band.
To detect an MDCM signal while acquiring the diversity gains in a receiving stage, an ML (Maximum-Likelihood) scheme may be used. Since a SISO system can detect imaginary and real parts, a SISO system can be implemented without greatly increasing complexity even if a complex ML scheme is used. However, when MDCM technology is applied to a 2×2 MIMO system, the 2×2 MIMO system might not separately detect real and imaginary parts, and the 2×2 MIMO system performs detection similar to a 4×4 MIMO system. Therefore, a 2×2 MIMO system performs detection through an ML scheme or an SD (sphere decoding) scheme used in an existing MIMO system.
Since an ML scheme is complex, an ML scheme might not be implemented in a real system. Although an SD scheme is less complex than an ML scheme, an SD scheme is sequential due to a depth-first scheme. Accordingly, an SD scheme is not suitable for an MDCM-applied 2×2 MIMO system that must process data at a high data rate.
In addition, the MB-OFDM UWB physical layer standard recommends the use of MDCM modulation technology together with LDPC (low-density parity check) channel coding technology. Accordingly, LDPC channel decoding must be performed at the receiver of an MB-OFDM system. Since LDPC channel decoding is performed based on a log-likelihood ratio (LLR) for each encoded bit, a detector for the 2×2 MIMO system employing MDCM transmits a correct LLR value to an LDPC decoder.
A detection scheme that generates an LLR value and transmits the LLR value to a channel decoder is referred to as a soft-decision detection scheme. Although an ML scheme is applicable to a soft-decision detection scheme, and optimal performance can be achieved, the complexity of an ML scheme inhibits implementation. In addition, since an LLR value for soft-decision might not be generated through an SD detection scheme, SD detection impossible may not be possible.